In the book, the private eye is sort of a macho guy who can cook, but his girlfriend can't cook. He tells her to add more coffee (to make it stronger) and a pinch of salt (who knows, but a post below suggests it might be to cut the bitterness).

The result is strong coffee he likes and coffee she thinks is too strong (and salty -- though maybe she's just imagining it ... which means I'm now speculating on what a fictional character might be imagining ... which is weird)

An early method for decaffeinating coffee beans involved soaking the unroasted beans in salt water to extract the caffeine. So adding salt to the grounds before they are brewed actually makes sense if salt is being added in order to increase the caffeine level. I'm not sure if that small an amount of salt would make a difference. I hope it does because it would give me a way to boost the caffeine in my cold brewed coffee.

My housekeeper when I was young always put salt in her coffee. Always. For a while we thought she just confused it with the sugar (our house doesn't have labels on the jars) so we tried to tell her and she said that she prefers salt. I can never bring myself to try it though.

I used to add a pinch of salt and a shake of cinnamon to the coffee grounds on a daily basis...( before brewing) it cuts the bitterness and brings out the coffee flavor...you don't taste the salt at all..now that better coffee beans are available I don't add salt at all..

I'm no cook, but by osmosis (my wife watches Food Network) I thought I learned that sometimes salt is used in small quantities to enhance other flavors, even when you can't taste the salt. This reminds me of that.

Yes. I think that's exactly right. When I saute garlic in olive oil, I always add a little salt... it seems to diffuse the flavor throughout whatever dish I'm using it in. I worked with a chef who insisted on a little salt in virtually everything he made - albeit in the beginning stages. And his dishes were never salty. But very, very flavorful.

A good cup of coffee doesn't need any doctoring at all, but sometimes you get a cup that's more bitter than you like. Some people add sweetener to counteract the bitterness, but salt is actually much more effective at decreasing your perception of bitterness. Try it! It's just a matter of taste, really. If it's legit to put sugar in your coffee, it's equally legit to use salt. In the same vein, if you find grapefruit too bitter on its own, try sprinkling salt on it instead of sugar.

I believe that this originated while using chichory because of the bitterness, I have done it also with regular french roast and can't report anything remarkably different. I used to put it into the ground coffee prior to brewing.